1. Field of the Invention
The present invention relates generally to improvements in power tools and, more particularly, but not by way of limitation, to power tongs used to turn pipes and the like to assemble and disassemble drill strings and the like in oil well drilling operations.
2. Brief Description of the Prior Art
In drilling an oil well, the drill string is lengthened as the depth of the well is increased by the addition of pipes to the upper end of the string. Removal of the string from the well similarly involves the disassembly of the string by removal of pipes from the upper end of the string. The pipes are provided with screw fittings so that the assembly and disassembly operations involve the turning of a pipe while the remainder of the drill string is held in a fixed position.
Because of the size of drill pipes, power tongs have been developed to facilitate the turning of such pipes and various types of tongs are known in the art. While these types of tongs vary in construction and use, they include certain basic components and their use imposes certain basic requirements on the capabilities of these components. In general, a power tongs will include a case which is suspended via a leveling mechanism so that the tongs can be aligned with the pipe prior to use. A rotor is mounted on the case along with a motor for driving the rotor and a jaw assembly is supported on the rotor for gripping the pipe so that the pipe can be turned via a rotation of the rotor. To accomplish this purpose, the jaw assembly is designed to first grip the pipe when the rotor is turned in the direction in which the pipe is to be turned and, then, to transmit the force of the motor to the pipe. To release the pipe, the rotor is turned in the opposite direction and the jaw assembly is constructed to release the pipe when the direction of rotation of the rotor is reversed.
The need for the power tongs to grip or release a pipe depending upon the direction in which the rotor is turned, along with the need to turn the pipe in one direction for assembly of a drill string and in an opposite direction for disassembly of the drill string, presents the basic problem of power tongs construction. At times, it is necessary that the jaws of the tongs grip the pipe in response to a clockwise direction of rotation of the rotor relative to the pipe and release the pipe in response to a counter clockwise rotation of the rotor. At other times, it is necessary for gripping to occur upon a counter clockwise rotation of the rotor and for release to occur upon clockwise rotation of the rotor. Various schemes have been employed to provide power tongs with selectivity in the direction of rotation of the rotor which is to cause a pipe to be gripped or released. Thus, the power tongs disclosed in U.S. Pat. No. 2,703,221 issued Mar. 1, 1955 to Gardner, utilizes inversion of the tongs to achieve such selectivity. That is, the tongs disclosed by Gardner is constructed such that the jaws will grip a pipe when the rotor is turned in one direction relative to the case of the tongs and will release the pipe when the rotor is turned in the opposite direction relative to such case. The orientation of the tongs relative to the pipe then determines whether the gripping direction will turn the pipe in a clockwise or counter clockwise direction relative to the drill string.
While the tongs inversion scheme disclosed by Gardner solves the problem of providing a power tongs with selectivity in the direction of rotation of the rotor, relative to the pipe, which will cause the pipe to be gripped, this scheme can give rise to another problem. As has been noted by Lundeen in U.S. Pat. No. 2,435,972 issued Feb. 17, 1948, it is important that a tongs be leveled prior to the gripping of a pipe thereby. It has been found that such a leveling after inversion can be a time consuming and, consequently, expensive chore.
A scheme for selecting the direction of rotation of the rotor which will cause the jaws to grip the pipe, and which does not suffer from problems involving repeated leveling of the tongs, it is used in a power tongs manufactured by B. J. Hughes, Inc. of Houston, Tex. In this tongs, a jaw and anvil are pivotally mounted on a jaw support which, in turn, is mounted on the rotor of the tongs. A frictional drag is applied to the jaw support so that the jaw support is held fixed during the initial rotation of the rotor. The jaw has a portion which projects into the rotor between two rollers disposed therein so that, when the rotor is turned in one direction, one of the rollers will engage the jaw to pivot the jaw toward the pipe which is disposed within central apertures of both the rotor and the jaw support. After the pipe has been engaged, continued rotation of the rotor turns the jaw support and the pipe. To release the pipe, the rotor is turned in the opposite direction to cause the other of the two rollers to engage the jaw and pivot the jaw away from the pipe. Selectivity in the direction of rotation of the rotor to cause the jaw to grip the pipe against the anvil is provided by mounting the jaw and anvil via pins so that the jaw and anvil can be interchanged on the jaw support and by providing the rotor with two pairs of rollers on opposite sides of the rotor. Both the jaw support and the rotor have a mirror symmetry so that the selection of the pair of rollers to be utilized to pivot the jaw determines the direction the jaw will be pivoted, toward or away from the pipe, when the rotor is turned in a given direction.
While tongs which utilize jaw and anvil interchangeability to achieve selectivity in the direction of rotation of the rotor to cause gripping of a pipe to be turned by the tongs eliminates the need for leveling the power tongs each time the direction in which a pipe is to be turned is reversed, such tongs introduce a new problem. The rollers used to actuate the jaw of such tongs are subjected to considerable battering in the operation of the tongs and, as a result, can become jammed in the rotor after a period of operation. This jamming causes the rollers to slide, rather than roll, on the jaw when the tongs is used to give rise to excessive wear of the jaw. Accordingly, when a roller becomes jammed, it is necessary that the roller be replaced if expensive subsequent repair of the tongs is to be avoided. In power tongs heretofore known, such replacement of the rollers has required that the tongs be disassembled to obtain access to the rollers.